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Lesson 3: North America WeatherSystems

Pages 206-207

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Dominant factors that produce seasonal weather

Temperature Wind Pressure Moisture

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Define Weather System

Weather system is a set of temperature, wind, pressure, and moisture conditions for a certain region that moves as a unit for a period of days.

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Define Air Mass

Air mass is a large body of air in which the temperature and moisture content at a specific altitude are fairly uniform.

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Types of Air Masses

1. Maritime

2. Continental

3. Polar

4. Tropical

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Air Masses: Maritime

Maritime means it is formed over water

It forms over the Atlantic or Pacific Oceans

It soaks up water from the ocean The air will be wet

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Air Masses: Continental

Continental means it formed over the land

Very little moisture over landAir will be dry

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Air Masses: Polar

Polar means it formed over the cold regions of the North Pole

Covered in ice and snowAir over this area is cold

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Air Masses: Tropical

Tropical means the air mass is formed in the warm tropical regions

Air mass gains a lot of energy from the sun

Air over this region is warm

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Air Masses

Polar (cold) Tropical (Warm)

Maritime (wet)

Wet and cold Wet and warm

Continental (Dry)

Dry and cold Dry and warm

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Types of Air Masses

1. Maritime Polar

2. Continental Polar

3. Maritime Tropical

4. Continental Tropical

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Maritime Polar

Polar maritime air masses are cold and wet and usually bring fog and cool temperatures in the summer and heavy snow and very cold temperatures in the winter.

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Continental Polar

Continental Polar air masses form over northern Canada and bring cold, dry air in the winter.

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Maritime Tropical

Maritime Tropical air masses are warm and wet and usually bring warm "muggy" rain in the summer and slushy snow and mild temperatures in the winter.

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Continental Tropical

Continental Tropical air masses form over Mexico and bring dry and hot air in the summer.

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Define Front

When two air masses meet the boundary that forms between them is called a front.  The weather conditions at the front is usually very stormy and unsettled.  Fronts mean rain!

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Four Types of Fronts

1. Warm Fronts

2. Cold Fronts

3. Occluded Fronts

4. Stationary Fronts

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Warm Fronts

A warm front is caused by a warm air mass moving slowly up a stationary (stopped) cold air mass.  As the warm air slowly rises and cool showers (or flurries) develop.  Conditions are somewhat calm.  The air that follows behind the warm front is warm and humid (damp). 

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Cold Fronts

Cold air mass pushes underneath a warm air mass.  The warm air is forced up.  As the warm air rises it

begins to cool and loses its moisture.  Forms clouds at first but then develops into heavy

rain (or snow) and stormy weather.  The air that follows behind the cold front is cool and dry. 

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Occluded Fronts

An occluded front is formed when a cold air mass catches up with a slower moving warm air mass.  The cold air mass slowly pushes the warm  air up creating gentle precipitation.  Storms caused by an occluded front are generally gentle.

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Stationary Fronts

Stationary fronts form when two air masses collide and stall (stop).

The front slowly moves through over a long period of time.  Stationary fronts usually bring rain / or snow for long periods of time and the conditions stay quite calm.  It is common to have rain, drizzle and fog lasting for days!